modules: move 'content' and 'anchors' to MODULE_COMMON_ATTRS

All modules implement these. The only one that differs is the i3
module. It still implements them, but 'content' uses an internal
verifier function.

modules/mpd.c

@@ -610,8 +610,6 @@ verify_conf(keychain_t *chain, const struct yml_node *node)
     static const struct attr_info attrs[] = {
         {"host", true, &conf_verify_string},
         {"port", false, &conf_verify_int},
-        {"content", true, &conf_verify_particle},
-        {"anchors", false, NULL},
         MODULE_COMMON_ATTRS,
     };